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Register a new userX-ray spectral and flux variability of the microquasar GRS 1758-258 on timescales from weeks to years
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Maria Hirsch
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We present the spectral and timing evolution of the persistent black hole X-ray binary GRS 1758-258 based on almost 12 years of observations using the Rossi X-ray Timing Explorer Proportional Counter Array. While the source was predominantly found in the hard state during this time, it entered the thermally dominated soft state seven times. In the soft state GRS 1758-258 shows a strong decline in flux above 3 keV rather than the pivoting flux around 10 keV more commonly shown by black hole transients. In its 3-20 keV hardness intensity diagram, GRS 1758-258 shows a hysteresis of hard and soft state fluxes typical for transient sources in outburst. The RXTE-PCA and RXTE-ASM long-term light curves do not show any orbital modulations in the range of 2 to 30 d. However, in the dynamic power spectra significant peaks drift between 18.47d and 18.04d for the PCA data, while less significant signatures between 19d and 20d are seen for the ASM data as well as for the Swift/BAT data. We discuss different models for the hysteresis behavior during state transitions as well as possibilities for the origin of the long term variation in the context of a warped accretion disk.
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The family links between radio galaxies and microquasars have been strongly strengthened thanks to a new common phenomenon: the presence of extended winged features. The first detection of such structures in a Galactic microquasar, recently reported in Nature Communications (http://rdcu.be/zgX8), widens the already known analogy between both kinds of outflow sources (Marti et al. 2017). This observational result also has potential implications affecting the black hole merger scenarios that contribute to the gravitational wave background.
The XMM-Newton X-ray observatory pointed the galactic black hole candidate and microquasar GRS 1758-258 in September 2000 for about 10 ks during a program devoted to the scan of the Galactic Center regions. Preliminary results from EPIC MOS camera data are presented here. The data indicate that the source underwent a state transition from its standard low-hard state to an intermediate state. For the first time in this source the ultra-soft component of the accretion disk, which black hole binaries display in intermediate or high-soft states, was clearly detected and measured thanks to the high spectral capabilities of XMM-Newton.
The results of GRANAT/SIGMA hard X-ray observations of GRS 1758-258 in 1990-1998 are presented. The source lies at ~5arcdeg from the Galactic Center and was within the SIGMA field of view during the GRANAT surveys of this region. The total exposure time of the Galactic Center was 11x10^6 s. The regular SIGMA observations revealed strong variability of the source: the 40-150 keV flux varied at least by a factor of 8 on a time scale of a year, between less than 13 mCrab and ~100-110 mCrab. The average flux was ~60 mCrab in 1990-1998. The sources spectrum is well fitted by a power law with a photon index alpha ~1.86 in the energy range 40 to 150 keV and becomes steeper at energies above ~100 keV. The radio and hard X-ray properties of GRS 1758-258 are similar to those of another Galactic Center source, 1E1740.7-2942. GRS 1758-258 and 1E1740.7-2942 are the two brightest hard X-ray sources in the Galactic Center region. Both sources have radio jets, similar X-ray luminosities (~10^37 erg/s), and spectra, and exhibit variations in the hard X-ray flux on long times scales by a factor of ~10 or more . In contrast to most of the known black hole candidates, which are X-ray transients, GRS 1758-258 and 1E1740.7-2942 were detected by SIGMA during most of the observations in 1990-1998. Assuming that this behavior of the sources implies the suppression of accretion-disk instability in the region of partial hydrogen ionization through X-ray heating, we impose constraints on the mass of the optical companion and on the orbital period of the binary system.
Context. Understood to be a microquasar in the Galactic center region, GRS 1758-258 has not yet been unambiguously identified to have an optical/near-infrared counterpart, mainly because of the high absorption and the historic lack of suitable astrometric stars, which led to the use of secondary astrometric solutions. Although it is considered with 1E 1740.7-2942 as the prototypical microquasar in the Galactic center region, the Galactic origin of both sources has not yet been confirmed. Aims. We attempt to improve previous astrometry to identify a candidate counterpart to GRS 1758-258. We present observations with the Gran Telescopio de Canarias (GTC), in which we try to detect any powerful emission lines that would infer an extragalactic origin of this source. Methods. We use modern star catalogues to reanalyze archival images of the GRS 1758-258 field in the optical and near-infrared wavelengths, and compute a new astrometric solution. We also reanalyzed archival radio data of GRS 1758-258 to determine a new and more accurate radio position. Results. Our improved astrometric solution for the GRS 1758-258 field represents a significant advancement on previous works and allows us to identify a single optical/near-infrared source, which we propose as the counterpart of GRS 1758-258. The GTC spectrum of this source is however of low signal-to-noise ratio and does not rule out a Galactic origin. Hence, new spectral observations are required to confirm or discard a Galactic nature.
We report on an XMM-Newton Reflection Grating Spectrometer observation of the black hole candidate and Galactic microquasar GRS 1758-258. The source entered a peculiar off/soft state in late February, 2001, in which the spectrum softened while the X-ray flux -- and the inferred mass accretion rate -- steadily decreased. We find no clear evidence for emission or absorption lines in the dispersed spectra, indicating that most of the observed soft flux is likely from an accretion disk and not from a cool plasma. The accretion disk strongly dominates the spectrum in this lower-luminosity state, and is only mildly recessed from the marginally stable orbit. These findings may be difficult to explain in terms of advection-dominated accretion flow, or ADAF models. We discuss these results within the context of ADAF models, simultaneous two-flow models, and observed correlations between hard X-ray flux and jet production.
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